HP 86/87 Matrix Rom Manual

[patrice]

As far as I remember.
There is a set of ROMs for HP 83 and 85 and a set of ROMs for HP 85B, 86 and 87.
The main reason of those 2 sets of ROMs is that the 83/85 have 16 bits memory address space and the CPU can access everywhere.
Contrary, the 85B/86/87 have 24 bits memory address space and access of memory above the first 64KB is done with 2 special pointers.
the difference between direct memory access and pointer memory access takes a small amount of space in CPU code to do the same thing. That small amount over a ROM can be enough to make that the code don't fit in the 8KB of the ROM, thus needing 2 ROMs.